The footage showing chickens in cramped cages and being slammed into bins is part of an escalating war with the food sector.

The Humane Society of the United States has released undercover video footage shot at two of the nation's largest egg farms showing workers slamming chickens into metal bins and dead birds littering cages -- the latest salvo in an escalating war between the food sector and the country's leading animal-rights organizations.

At stake, both sides said, is regulating how livestock are treated and how Americans' food is produced.

Since California voters passed Proposition 2 in 2008, Humane Society officials have ramped up their campaigns to alter state laws regarding animal welfare. They're reaching out to young people, including a presentation at last month's National 4-H Conference in Washington, where they encouraged teenage future farmers to treat livestock with respect.

The organization has also been buying chunks of stock in publicly traded food companies, in part to be able to introduce shareholder resolutions and pressure company executives to alter their purchasing decisions.

The strategy has worked. Companies including Wendy's, Sonic Corp. and the parent company of the IHOP and Applebee's restaurant chains have all started shifting to using cage-free eggs, according to Humane Society officials. Wal-Mart Stores Inc., the nation's largest grocer, said in February that the eggs sold under its store label were now cage-free.

"Companies generally don't want graphic information about how their suppliers abuse animals for their products sent to every investor and the media," said Paul Shapiro, head of the group's Factory Farming campaign.

But the farmers are fighting back. In recent months, agribusiness lobbyists and farm groups have bombarded companies sympathetic to the Humane Society with letters asking them to halt donations to the group.

"HSUS seeks to remove meat from our dinner tables, leather goods from our closets, animals from zoos and circuses and eventually -- pets from our families," Kansas Farm Bureau President Steve Baccus wrote in a letter to Bank of America Corp. posted on the bureau's website. The Humane Society, he wrote, is "a powerful, well-funded activist organization pursuing what most reasonable observers would consider an extreme anti-animal agenda."

In Missouri, livestock farmers are worried that a Humane Society-backed ballot initiative to curtail so-called puppy mills could lead to attacks on livestock interests. They have spurred a bid for a constitutional amendment that would ensure the public's right "to raise animals in a humane manner . . . without the state imposing an undue economic burden on their owners."

And in Ohio, a state animal welfare standards board was created in an effort by farm interest groups and lawmakers to block future ballot measures that might affect livestock operations. But that hasn't deterred the Humane Society. The group is collecting signatures to put a bill before voters this November that's similar to the one passed in California.

"They are huge. They are influential, and they are our biggest concern on the impact of our ability to domestically produce food," said Kay Johnson Smith, executive vice president of the Animal Agriculture Alliance, a trade group that represents livestock farmers and ranchers.

The egg-farm footage released Wednesday was shot surreptitiously over the last two months inside Iowa facilities owned by Rose Acre Farms and Rembrandt Enterprises. It was taken by a Humane Society volunteer, who had landed work at four Iowa hen operations.

Among other things, the video footage showed chickens crammed into cages so crowded that the animals couldn't move and their talons couldn't touch the floor; chickens held in battery cages above manure pits that allegedly hadn't been regularly cleaned; and a worker stuffing birds into a euthanizing chamber with such force that the thunk of the animals' heads hitting the metal exterior could be heard.

One worker, whose face was blacked out on the footage, told the undercover videographer that disease had killed many birds at one location: "It was nothing to pull 5,000 out of there a day," the worker is filmed as saying. "And that's all we did for about two weeks straight, is just pull dead birds."

Tony Wesner, executive vice president of Rose Acre Farms in Seymour, Ind., said Wednesday morning that the company "doesn't condone inhumane treatment" of its livestock. "Anyone violating our standards would be immediately terminated," Wesner said.

Representatives of Rembrandt Enterprises, based in Rembrandt, Iowa, could not be reached for comment.